test: Add a way to detect a test that breaks another

When running unit tests, some may have side effects which cause a
subsequent test to break. This can sometimes be seen when using 'ut dm'
or similar.

Add a new argument which allows a particular (failing) test to be run
immediately after a certain number of tests have run. This allows the
test causing the failure to be determined.

Update the documentation also.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/spl.c

@@ -132,7 +132,7 @@ void spl_board_init(void)
 		int ret;
 
 		ret = ut_run_list("spl", NULL, tests, count,
-				  state->select_unittests, 1, false);
+				  state->select_unittests, 1, false, NULL);
 		/* continue execution into U-Boot */
 	}
 }